diff --git a/types/fetch.d.ts b/types/fetch.d.ts
index fa4619c9182..16a4c1654f4 100644
--- a/types/fetch.d.ts
+++ b/types/fetch.d.ts
@@ -50,7 +50,7 @@ export interface SpecIterable<T> {
   [Symbol.iterator](): SpecIterator<T>;
 }
 
-export type HeadersInit = string[][] | Record<string, string | ReadonlyArray<string>> | Headers
+export type HeadersInit = string[][] | Record<string, string | ReadonlyArray<string> | undefined> | Headers
 
 export declare class Headers implements SpecIterable<[string, string]> {
   constructor (init?: HeadersInit)